First Teeth Grew on Outside of Body

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The fictional Cheshire cat's smile seemed to have a life of its
own, outside of the cat's body, and now new research suggests the
world's first teeth grew outside of the mouth before later moving
into the oral cavity.

The study, published in the Journal of Vertebrate
Paleontology, supports what is known as the "outside-in"
hypothesis of tooth evolution. The first teeth and smile,
however, did not belong to a cat, but likely were flashed by
small and spiny shark-like fishes.

"The first smile would probably have been a prickly one, with
many tiny teeth that looked like pointy cheek scales, and other
small tooth-like scales wrapping around the lips onto the outside
of the head," co-author Mark Wilson told Discovery News.

For the study, Wilson, a professor in the Department of
Biological Sciences at the University of Alberta, and his
colleagues studied animals called ischnacanthid
acanthodians, an extinct group of fish that resembled
sharks. They lived during the Early Devonian period, which lasted
from 416 to 397 million years ago.

The researchers determined that head scales from these fish were
in transition, evolving from scales to teeth. The pointy
structures were identified on the lips of the fish. This
discovery helps to negate the “inside out” theory of tooth
evolution, which holds that the first teeth emerged from
structures in the pharynx progressing into the mouth.

Project leader Stephanie Blais, a University of Alberta
researcher, told Discovery News that "our findings support the
idea that teeth evolved from modified pointed scales on the mouth
margins (lips) as we see in Obtusacanthus," one of the
prehistoric fish included in the study. All of the analyzed fish
specimens were excavated at the Man on the Hill site in the
Mackenzie Mountains of Canada.

As to why teeth first evolved, Blais said they "would have
conferred a major advantage in terms of food acquisition. Pointed
scales near the margins of their mouths would have helped them
grasp prey and hang on to it until they could swallow it whole."

Such prey consisted of "probably whatever they could swallow,"
co-author Lindsay MacKenzie of the University of Montana’s
Department of Geosciences told Discovery News. Based on
fossilized stomach contents and other evidence, their primary
prey probably consisted of arthropods, including crustaceans, as
well as a variety of soft-bodied creatures and fish.

Blais said jaws, which must have evolved earlier, and the
tooth-like formations "allowed fishes to change from a
filter-feeding or mud-grubbing more passive lifestyle to one of
active predation."

The world's first aggressive conflicts also may have arisen at
this point, since the move from passive feeding to hunting led to
what Blais termed "the very first evolutionary arms race" among
vertebrates, with some becoming predators and others becoming
prey.

The first teeth also probably first arose in sets, and not just
as a single tooth here and there. MacKenzie explained that
"scales and teeth exist in developmental fields in which similar
developmental processes generate many similar tooth or scale
elements." Since the first teeth likely arose from these scales,
they then to some extent mirrored the prior scale groupings,
emerging as sets of teeth.

There is a connection between human teeth and these first fish
teeth.

"Because teeth would have allowed vertebrates to become more
efficient predators, and eventually more efficient herbivores,
they were retained and passed down through generations in most
groups," Blais said.

While humans and other animals with teeth share this initial fish
connection, a lot of evolution, as well as inter-species battles,
occurred over the millions of years.

"Interactions between predators and prey through time only
increased their (teeth's) importance, leading to highly
specialized teeth, such as those in mammals," Blais said.